Optical module and optical switch constituting the same

ABSTRACT

An optical module controls its output characteristics electrically and an optical switch constitutes the optical module. An optical waveguide circuit (PLC) and an electronic circuit (IC) for driving the PLC are mounted on the same substrate. The IC is composed of a bare chip to be molded afterward. Wiring of the IC is grouped and integrated on the PLC substrate to achieve higher density and miniaturization of the optical module.

This application claims priority from Japanese Patent Application Nos.2002-323044 filed Nov. 6, 2002 and 2002-341334filed Nov. 25, 2002, whichare incorporated hereinto by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a 1×N optical switch, an integratedoptical switch including optical waveguide switches coupled in amulti-stage. The present invention further relates to an optical modulethat electrically controls its output characteristics and is applicableto an optical communication system, to an optical switch and an opticalmatrix switch constituting the optical module.

2. Description of the Related Art

To cope with a sharp increase in traffic volume in data communicationnetworks typified by the Internet recently, a large increase in capacityis being carried out by using optical transmission technology such aswavelength division multiplexing (WDM) transmission. Recently, theoptical transmission technology is only applied to point-to-pointoptical links interconnecting nodes, but not applied to the processingwithin each node which is still carried out electrically. As the largetransmission capacity more increases, the electrical processing posesproblems of a slow increase in throughput and a sharp increase in cost.An optical cross-connect system and optical add drop system usingoptical switches can implement cut-through process by handling almostall the optical signals within a node optically, thereby being able todramatically increase the throughput with reducing the cost. Thus, theoptical switch is an essential device for constructing a large capacity,flexible communication network at a lower cost.

The optical switches are implemented in various types. Above all,optical waveguide switches are excellent in mass productivity andminiaturization. Conventionally, optical waveguides have been fabricatedfrom a variety of materials. For example, silica-based opticalwaveguides formed on silicon substrates are characterized by a low loss,high stability and good matching with optical fibers. In addition, manydifferent varieties of optical components typified by arrayed waveguidegrating (AWG) multiplexer/demultiplexers are actually used.

A 1×N optical switch, one of the integrated optical switches, is anoptical switch enabling one input port to be connected to any one ofoutput ports. For example, in a selecting switch for a monitoringdevice, a selecting switch from light sources, or an opticalcross-connect system, it is applicable to important optical switchessuch as an N×N optical matrix switch consisting of a combination of aplurality of 1×N optical switches. As for silica-based waveguides, a 1×Noptical switch has been implemented in a 1×128 scale.

FIG. 1 shows an arrangement of a conventional tree type 1×8 opticalswitch. It is implemented by connecting two 2×2 optical switch units tooutputs of a 2×2 optical switch unit connected to an optical input port,and successively cascading 2×2 optical switch units in a 3-stageconstitution. The 2×2 optical switch unit at an a-stage, b-row positionis represented by a-b (e.g. 3-2). The third stage 2×2 optical switchunits have their output waveguides connected to optical output ports viagate optical switches. The c-th gate optical switch from the top isdenoted by G-c (e.g. G-3). Each 2×2 optical switch unit can connectselectively one of the two inputs to any one of the two outputwaveguides. Thus connecting the 2×2 optical switch units in cascade withmulti-stage enables the whole to function as a1×N optical switch. Thegate optical switches connected to the optical output ports improve theextinction ratio by carrying out ON/OFF operation.

FIG. 2 shows an arrangement of a conventional tap type 1×8 opticalswitch. A 2×2 optical switch unit connected to an optical input port hasits first output waveguide connected to an input waveguide of thenext-stage 2×2 optical switch unit, and its second output waveguideconnected to an input waveguide of a gate optical switch connected to anoptical output port. Thus, eight 2×2 optical switch units are connectedin an 8-stage constitution. Although FIG. 1 shows an example of the treetype arrangement, a combination of the tree type construction and taptype construction is also possible. The tree type construction issuperior in reduction in size and loss of the switch circuit. On theother hand, the tap type construction is superior in lower consumptionof the switch circuit. For example, arrangements of the optical switchesmaking use of these features of each type are disclosed in T. Goh, etal., “Large-scale integrated silica-base thermo-optic switches”, NTTReview, Vol. 13, No. 5, pp. 18–25, 2001.

FIGS. 3A–3C shows an arrangement of a conventional 2×2 optical switchunit. FIG. 3A is a plan view of a 2×2 optical switch unit 1100 built onsilica-based waveguides. FIG. 3B is a cross-sectional view taken alongthe line A–A′ of FIG. 3A, and FIG. 3C is a cross-sectional view takenalong the line B–B′ of FIG. 3A. The 2×2 optical switch unit 1100 is aMach-Zehnder interferometer (called “MZI” from now on) type 2×2 opticalswitch including two arm waveguides 1103 a and 1103 b. The two armwaveguides include a thermooptic phase shifter utilizing thin-filmheaters 1101 a and 1101 b, and have their both ends connected with 3 dBcouplers 1102 a and 1102 b.

An MZI type 2×2 optical switch having two arm waveguides 1103 a and 1103b of an equal length is called a symmetric type MZI. In contrast, an MZItype 2×2 optical switch having two arm waveguides 1103 a and 1103 b withan optical path difference of half wavelength is called an asymmetrictype MZI. According to a known interference principle, the symmetrictype MZI propagates light along the crossed path (port 1A→port 2B) whenthe thermooptic phase shifter is not driven, and along the bar path(port 1A→port 2A) when the thermooptic phase shifter is driven becauseof the optical path difference of half wavelength caused by thethermooptic effect.

In addition, the optical path is continuously shifted from the crossedpath to the bar path by continuously varying the optical path differencebetween the two arm waveguides from zero to a half wavelength bycontrolling the driving current supplied to the thin-film heaters 1101 aand 1101 b. In other words, the MZI type 2×2 optical switch operates notonly as an ON/OFF switch, but also as a continuously adjustable analogswitch between transmission and interruption of light. Accordingly, theMZI type 2×2 optical switch can be used as an attenuator or an opticalbranching circuit for carrying out multicast or broadcast by adjusting adistribution ratio between the crossed path and the bar path.

The gate optical switches as shown in FIGS. 1 and 2 use asymmetric typeMZIs. The asymmetric type MZIs propagate light along the bar path (port1A→port 2A) when the thermooptic phase shifter is not driven, but alongthe crossed path (port 1A→port 2B) when the thermooptic phase shifter isdriven because the optical path difference of half wavelength iscanceled out by the thermooptic effect. Thus, as for the gate opticalswitches, using the asymmetric type MZI makes it possible to economizethe power consumption, and to take full advantage of the cross port witha higher extinction ratio.

Thermooptic switches using the silica-based waveguides are fabricated bycombining a glass film deposition technique such as flame hydrolysisdeposition (FHD) or chemical vapor deposition (CVD), with a microetching technique such as a reactive ion etching method (RIE). Morespecifically, a glass film of an under cladding layer is formed on asubstrate such as a silicon wafer, followed by depositing a core layerwith a refractive index slightly higher than that of the cladding layer.Subsequently, a core pattern is formed by a micro etching technique,followed by depositing a glass film to be shaped to an over claddinglayer. Finally, thin-film heaters of the thermooptic phase shifters andwiring for supplying power to them are formed, thereby fabricating anoptical switch chip. The optical switch module is completed byconnecting power supply lines and optical fibers to the optical switchchip, and by packing it into a case with a radiator fin.

A 1×128 optical switch fabricated by using the tree type arrangement asshown in FIG. 1 and the 2×2 optical switch units as shown in FIGS. 3A–3Ccan achieve superior characteristics with the average insertion loss of3.7 dB and average ON/OFF extinction ratio of 50.8 dB (For example,refer to T. Watanabe et al., “Silica-based PLC 1×128 thermo-opticswitch”, Proc. 27th ECOC'01, Tu.L.1.2, Amsterdam, 2001).

The conventional 1×N optical switch module, however, has a problem ofrequiring an enormous number of driving circuits of the thermoopticphase shifters, that is, the driving current supply circuits for thethin-film heaters. FIG. 4 shows the driving current supply circuits ofthe conventional tree type 1×8 optical switch. To control the individual2×2 optical switch units, the power supply lines 11–14 of the drivingcurrent supply circuits, which connects to the 2×2 optical switch unitsindividually, are connected to analog adjustable driving power supplycircuits (not shown in FIG. 4), and the driving current supply circuitsare connected to control lines. The power is supplied to one of the twothin-film heaters. The control lines are not shown in FIG. 4 to simplifythe drawing.

As for the 1×N optical switch, the number of the driving power supplycircuits required for the tree type construction is given by thefollowing expression.2^((log) ₂ ^(N×1))−1

In contrast, the tap type construction requires 2N driving power supplycircuits. Accordingly, as for the 1×128 optical switch, the tree typeconstruction requires as many as 255 driving power supply circuits, andthe tap type construction requires 256 driving power supply circuits.

So far, the problem is described in that the number of the driving powersupply circuits is great.

Next, a problem will be described in that the area of the electricalwiring region in the PLC substrate is increasing for the above reason,and that the number of wires between the PLC substrate and driving ICassembly substrate is large.

FIG. 5 shows a conventional example of the optical switch module. Theupper half of FIG. 5 shows a substrate of a 1×128 optical switch 501using a thermooptic effect of a silica-based planar lightwave circuit(PLC), and the lower half of FIG. 5 shows an electrical wiring substrate521 on which ICs (integrated circuits) 525 for driving the opticalswitch are mounted. They together constitute the 1×128 optical switchmodule.

As for the PLC substrate, a plurality of 1×2 optical switches 503, eachof which consists of a 2-input 2-output optical switch unit, areconnected in a 7-stage tree, thereby configuring a 1×128 optical matrixswitch on the same substrate as shown in FIG. 5, (see T. Watanabe et.al., “Silica-based PLC 1×128 thermo-optic switch”, Proc. ₂₇ ^(th) ECOC'01, Tu.L.1, 2, Amsterdam, 2001).

Each 1×2 optical switch 503 of FIG. 5 uses the Mach-Zehnderinterferometer type 2×2 optical switch (MZI optical switch) as describedabove in connection with FIGS. 3A–3C.

Arranging the basic optical switches as shown in FIGS. 3A–3C in a treeconstruction as shown in FIG. 5 can implement the 1×128 optical switch501. The eighth stage of FIG. 5 is added to improve the extinction ratioby gate optical switches 505.

Each gate optical switch 505 consists of the asymmetric type MZI opticalswitch as described above.

FIG. 6 shows an arrangement of electrical driving circuits for heaters1101 a and 1101 b shown in FIGS. 3A and 3C. As shown in FIG. 6, theheaters on one-side arms of the MZIs of the 1×2 optical switches 503 and503 are connected to the driving analog power supply circuits 31 and 32which are adjusted such that they each supply an optimum voltage(current) for driving the MZIs. On the opposite ends of the heaters,electrical digital switches 41 and 42 are connected which are broughtinto conduction or out of conduction so as to turn on or off the opticalswitches 503 and 503. Since the total of 255 1×2 optical switches ispresent, there are 255 driving power supply circuits and 255 electricaldigital switches as well.

An actually fabricated 1×128 optical switch can achieve excellentcharacteristics of the average insertion loss of 0.4 dB, and the averageon/off extinction ratio of 40 dB.

Although the foregoing description is made by way of example of anoptical switch, a variable optical attenuator can be constructed in thesame manner by using the same MZI optical switches and by varying thevariation of the phase in an analog fashion. For example, the variableoptical attenuator is actually fabricated using the PLC. The variableoptical attenuator is an essential device for equalizing the lightintensity of the individual wavelengths of the signal light passingthrough the wavelength division multiplexing, and its demand has beenincreasing recently.

Other optical circuits growing in demand such as a dispersioncompensator, polarization mode dispersion compensator and gain equalizercan also be implemented by using the MZI optical switches and bycombining phase shifters and/or optical waveguides.

However, the foregoing 1×128 PLC thermooptic switch (PLC-TOSW) as shownin FIG. 5 has the following problems.

(1) The area of the electrical wiring region in the PLC substrate islarge scale and apt to increase.

(2) It requires a great number of analog power supply circuits fordriving, such as 255 power supply circuits in the foregoing example.

(3) The number of wire bondings between the PLC substrate and driving ICassembly substrate is increasing. The example of FIG. 5 requires 255electrode pads 511 and 515, and 255 wiring electrode pads for connectingto the upper driving circuits, thereby requiring the total of 510electrode pads.

(4) The inspection process requires a probe with a considerable numberof pins, which in turn requires high precision probe aligning equipment.

The foregoing problems will be described in more detail one by one.

(1) There are 255 thermooptic phase shifters (heaters) to be driven onthe substrate of FIG. 5, and their both ends are connected to the wiringelectrode pads 511 for electrical wires at the edge of the substrate viathe gold electrical circuits 507 and 509. Thus, it is necessary tolayout as many as 255 gold electrical circuits 507 and 509 on thesubstrate without an intersection. Accordingly, the area of theelectrical wiring sharply increases with the large scale of the opticalcircuit and the multichannels.

Next, the area of the electrical wiring will be estimatedquantitatively. As for gold electrical circuits for driving the Ta₂Nfilm heaters, considering that they are patterned on the opticalwaveguide substrate with a bend or warp, it is preferable that theyconsist of electric wiring in a single layer of the a gold thin film. Inaddition, considering the amount of the current required for driving theheaters, it is preferable that their width is about 50 μm, and the eachgap between the wires is about 50 μm. Estimating the area necessary todevelop the electrical wiring under the conditions, the 510 wiresrequire the wiring width of 51.2 mm. Since the size of the substrate is60 mm×60 mm in the example of FIG. 5, and the average wiring length isestimated to be 4 cm when the wiring is pulled to the edge of thesubstrate, the area of the electrical wiring region from the heaters tothe digital switches is estimated to be 20 cm². On the other hand, thearea of the optical circuit itself is reduced to 20 mm×60 mm=12 cm² byusing the core with a small permissible bending radius suitable forminiaturization and the cladding with a relative refractive indexdifference of 1.5%. Since the crossing layout of the optical circuitwith the electrical wiring itself is possible, it can be said that theminiaturization of the PLC optical switch substrate is limited by thearea of the electrical wiring region.

(2) The example of the 1×128 PLC thermooptic switch as shown in FIG. 5has on the substrate the 255 thermooptic phase shifters (heaters) to bedriven. Accordingly, it also requires as many as 255 driving (powersupply) circuits.

(3) In the foregoing (1), the 510 wires on the PLC substrate must beconnected one by one to the driving circuits 525 assembled on anothersubstrate. The connection between the substrates is usually performed bywire bonding. Assume that the wire bonding electrodes 511 and 515 are150 μm wide and the gap is 50 μm, then the pitch becomes 200 μm. Thus,the 510 wire bondings become as much as 104 mm wide.

Here, the following methods are taken to fix the PLC substrate on theelectronic circuit assembly substrate: a method of pasting the PLCsubstrate to a larger substrate including the electronic circuit; or amethod of placing the PLC substrate and electronic circuit substrate ona third substrate. Employing either method, which carries out the wirebonding at 200 degree Celsius, for example, can bring about thedifference in the contraction because of the thermal expansioncoefficient difference between each substrates, which can cause thestress to be imposed on wires 513, which can reduce the reliability.

(4) Furthermore, in the process of actually fabricating the module, aninspection process is essential which evaluates the opticalcharacteristics and electrical characteristics by bringing an electricalprobe into contact with the electrode pads from the outside and bydriving the heaters of the PLC substrate. To conduct the evaluation, itis necessary for the conventional example to bring the 510 electrodepads into contact with the electrical probe simultaneously, whichrequires a special and expensive electrical probe, and aligningequipment that enables the electrical probe to make contact with theelectrode pads on the PLC substrate in parallel at high precision.

Thus far, problems of the optical module are described whichelectrically connects the driving electrical circuit with the opticalcircuit by way of example of the 1×128 optical switch. These problemsoccur because the optical circuit and electrical circuit, which increasetheir size and the multichannel or multiport recently, are not optimizedin their entirety. For example, as for the example described above, theproblems result from the fact that the electrical circuit is notoptimized even though many heaters, which are present on the opticalwaveguide substrate to be driven by the electrical circuit, are placedin distributed locations because of typical circumstances of the opticalcircuit. Similar conditions apply to optical circuits other than the 1×Noptical switch, thereby constituting common problems to optical modulesthat electrically control the output characteristics. For example,similar problems can occur in an N×N matrix optical switch, a variableoptical attenuator and its arrayed module, a dispersion compensator, anda gain equalizer.

SUMMARY OF THE INVENTION

The present invention is implemented to solve the foregoing problems.Therefore a first object of the present invention is to provide a 1×Noptical switch capable of reducing the number of driving power supplycircuits without impairing the function of the switch.

A second object of the present invention is to provide an optical modulethat electrically connects a driving electrical circuit with an opticalwaveguide circuit, that is capable of miniaturizing an optical waveguidesubstrate and sharply reducing the number of gold wires which are likelyto impair the reliability, by reducing the area of the electrical wiringregion on the substrate, by reducing the number of the connection wiresfrom the PLC substrate to the outside, and by reducing the number ofanalog driving circuits, and father that is capable of simplifying thecharacteristic evaluation by eliminating an IC assembly substrate fordriving the optical switches or the optical variable attenuators, byreducing the module size, and by reducing the number of electrodeterminals of the probe.

To accomplish the first object, according to one aspect of the presentinvention, there is provided a 1×N optical switch comprising: a two-portoptical switch connected to an input waveguide, and capable ofcontinuously adjusting its optical output intensity from a first outputto a second output; a switch section including one or more two-portoptical switches connected in cascade to the output(s) of the two-portoptical switch, and having its outputs connected to N output waveguides,where N is an integer equal to or greater than three; a plurality ofgate optical switches, each of which is connected to one of the outputwaveguides and capable of continuously adjusting its optical output fromtransmission to interruption; a plurality of switch driving power supplycircuits for driving the two-port optical switches, the two-port opticalswitches being divided into groups, each of which includes only onetwo-port optical switch that is brought into conduction at a time, andthe two-port optical switches in a same group sharing one of the switchdriving power supply circuits; a gate driving power supply circuit fordriving the gate optical switches, all the gate optical switches sharingthe gate driving power supply circuit; and electrical digital switches,each of which is connected to one of the two-port optical switches orthe gate optical switches for interrupting a driving current from one ofthe driving power supply circuits.

To accomplish the first object, according to another aspect of thepresent invention, there is provided a 1×N optical switch comprising: atwo-port optical switch connected to an input waveguide, and capable ofcontinuously adjusting its optical output intensity from a first outputto a second output; a switch section including one or more two-portoptical switches connected in cascade to the output (s) of the two-portoptical switch, and having its outputs connected to N output waveguides,where N is an integer equal to or greater than three; a plurality ofgate optical switches, each of which is connected to one of the outputwaveguides and capable of continuously adjusting its optical output fromtransmission to interruption; a switch driving power supply circuit fordriving the two-port optical switches, all the two-port optical switchessharing the switch driving power supply circuit; a gate driving powersupply circuit for driving the gate optical switches, all the gateoptical switches sharing the gate driving power supply circuit; andelectrical digital switches, each of which is connected to one of thetwo-port optical switches or the gate optical switches for interruptinga driving current from one of the driving power supply circuits.

Preferably, power supply lines can be shared which connect the two-portoptical switches to the switch driving power supply circuits shared, andpower supply lines can be shared which connect the gate optical switchesto the gate driving power supply circuit shared.

To accomplish the first object, according to another aspect of thepresent invention, there is provided a 1×N optical switch comprising: atwo-port optical switch connected to an input waveguide, and capable ofcontinuously adjusting its optical output intensity from a first outputto a second output; a switch section including one or more two-portoptical switches connected in cascade to the output (s) of the two-portoptical switch, and having its outputs connected to N output waveguides,where N is an integer equal to or greater than three; a plurality ofgate optical switches, each of which is connected to one of the outputwaveguides and capable of continuously adjusting its optical output fromtransmission to interruption; a driving power supply circuit for drivingthe two-port optical switches and the gate optical switches, all thetwo-port optical switches and gate optical switches sharing the drivingpower supply circuit; and electrical digital switches, each of which isconnected to one of the two-port optical switches or the gate opticalswitches for interrupting a driving current from one of the drivingpower supply circuits.

Preferably, power supply lines can be shared which connect the two-portoptical switches and the gate optical switch to the driving power supplycircuit shared. The power supply lines can be shared on an opticalswitch chip in which the switch section, the plurality of gate opticalswitches and the power supply lines are integrated. Each of the two-portoptical switches can have its first output connected to one of theoutput waveguides and its second output unconnected or connected to aninput of one of other two-port optical switches to make the switchsection a tap type arrangement. The two-port optical switches or thegate optical switches can be each composed of a 2×2 optical switchhaving one of its input/output ports unconnected. The two-port opticalswitches or the gate optical switches can each consist of an opticalswitch using silica-based optical waveguides.

In the 1×N optical switch in accordance with the present invention, thetwo-port optical switches share driving power supply circuits fordriving them. Thus, it can reduce the number of the driving power supplycircuits without impairing the functions such as a unicast operation,thereby being able to miniaturize the optical switch.

In addition, the 1×N optical switch in accordance with the presentinvention can reduce the size of the optical switch chip, electricalcircuit substrate or optical switch module, and reduce the number of theelectrical connection terminals.

To accomplish the second object, according to one aspect of the presentinvention, there is provided an optical module comprising: an opticalwaveguide circuit; and a driving electronic circuit for providing theoptical waveguide circuit with a refractive index variation to modify anoutput characteristic, wherein the driving electronic circuit is mountedon a substrate of the optical waveguide circuit together with theoptical waveguide circuit.

Preferably, the driving electronic circuit can be mounted on thesubstrate of the optical waveguide circuit in the form of a bare chip.Wiring from the driving electronic circuit can be grouped and integratedon the substrate of the optical waveguide circuit. The optical waveguidecircuit can consist of an optical switch. The optical waveguide circuitcan consist of a variable optical attenuator. The optical waveguidecircuit can consist of a silica-based optical waveguide circuit.

Preferably, the optical waveguide circuit consists of a 1×N opticalswitch with one input and N outputs, where N is an integer equal to orgreater than three, wherein the 1×N optical switch comprises: one inputwaveguide placed on the substrate of the optical waveguide circuit; Noutput waveguides placed on the substrate of the optical waveguidecircuit; N gate optical switches, each of which is connected to one ofthe N output waveguides on the substrate of the optical waveguidecircuit, for controlling passing of light; and a plurality of 1×2optical switches placed between the input waveguide and the gate opticalswitches, for continuously switching its path in response to a level ofan electrical signal from the driving power supply circuit in thedriving electronic circuit, and wherein the plurality of 1×2 opticalswitches are divided into a plurality of groups, each of which isassigned one of the driving power supply circuits, and the opticalmodule further comprises electrical digital switches incorporated intointegrated circuits (ICs) for controlling levels of electrical signalssupplied from the driving power supply circuits to the plurality of 1×2optical switches.

Preferably, the optical waveguide circuit consists of an optical matrixswitch for linking mth input waveguide to lth output waveguide with anM×L optical cross-point switch, where M and L are an integer equal to orgreater than 2, and m and l satisfy relationships 1≦m≦M and 1≦l≦L,respectively, wherein the optical matrix switch comprises: M inputwaveguides placed on the substrate of the optical waveguide circuit; Loutput waveguides placed on the substrate of the optical waveguidecircuit; and an M×L optical cross-point switch placed between the Minput waveguides and the L output input waveguides on the substrate ofthe optical waveguide circuit, and consisting of a duplex type opticalswitch including 1×2 optical switches and 2×1 optical switches, each ofwhich continuously switching its path in response to the level of theelectrical signal fed from the driving electronic circuit, and whereinthe 1×2 optical switches and the 2×1 optical switches are divided into aplurality of groups, each of which is assigned one of driving powersupply circuits in the driving electronic curcuits, and the opticalmodule further comprises electrical digital switches incorporated intointegrated circuits (ICs) for controlling levels of electrical signalssupplied from the driving power supply circuits to the 1×2 opticalswitches and to the 2×1 optical switches.

According to the optical module in accordance with the presentinvention, it is possible in the optical circuit such as optical switchto reduce the size of the optical waveguide substrate by miniaturizingthe electrical wiring region on the substrate by the sharing; to sharplyreduce the number of the gold wires between the substrates, which arelikely to degrade reliability, by mounting the driving or control IC onthe optical waveguide substrate in the form of a bare chip; and the tominiaturize the module size by removing the IC assembly substrate fordriving the optical switch or optical variable attenuator.

Furthermore, the optical module in accordance with the present inventioncan greatly reduce the number of the electrode terminals (electrodepads) of the probe used for evaluating the characteristics of theoptical waveguide substrate in the inspection process of fabricating theoptical circuit, thereby being able to simplify the evaluation work andto cut costs.

The above and other objects, effects, features and advantages of thepresent invention will become more apparent from the followingdescription of embodiments thereof taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing a configuration of a conventionaltree type 1×8 optical switch;

FIG. 2 is a schematic diagram showing a configuration of a conventionaltap type 1×8 optical switch;

FIGS. 3A–3C are views showing a conventional 2×2 optical switch unit:FIG. 3A is a plan view, FIG. 3B is a cross-sectional view taken alongthe line A–A′ of FIG. 3A, and FIG. 3C is a cross-sectional view takenalong the line B–B′ of FIG. 3A;

FIG. 4 is a schematic diagram showing a configuration of driving currentsupply circuits of a conventional tree type 1×8 optical switch;

FIG. 5 is a schematic plan view showing a configuration of aconventional 1×128 PLC thermooptic switch;

FIG. 6 is a circuit diagram showing a configuration of driving currentsupply circuits of the conventional tree type 1×8 optical switch;

FIG. 7 is a schematic diagram showing a configuration of a firstembodiment of a tree type 1×8 optical switch in accordance with thepresent invention;

FIG. 8 is a circuit diagram showing driving current supply circuits ofthe first embodiment of the tree type 1×8 optical switch in accordancewith the present invention;

FIG. 9 is a schematic plan view showing an assembly of the firstembodiment of the tree type 1×128 optical switch module in accordancewith the present invention;

FIG. 10 is a schematic diagram showing a configuration of a secondembodiment of the tree type 1×8 optical switch in accordance with thepresent invention;

FIG. 11 is a block diagram showing a configuration of a third embodimentof a hybrid type 1×8 optical switch in accordance with the presentinvention;

FIG. 12 is a schematic diagram showing a configuration of a fourthembodiment of the tree type 1×8 optical switch in accordance with thepresent invention;

FIG. 13 is a circuit diagram showing driving current supply circuits ofthe fourth embodiment of the tree type 1×8optical switch in accordancewith the present invention;

FIG. 14 is a circuit diagram showing a configuration of an electricalcircuit including driving digital ICs, a basic arrangement of fifth toeighth embodiments in accordance with the present invention;

FIG. 15 is a time chart illustrating timing of a heater control signalapplied to the heater array of FIG. 14;

FIG. 16 is a schematic plan view showing a configuration of a 1×128 PLCthermooptic switch incorporating IC bare chips of a fifth embodiment inaccordance with the present invention;

FIG. 17 is an enlarged illustration showing details of the IC bare chipassembly of FIG. 16;

FIG. 18 is a schematic plan view showing an assembly incorporating an ICbare chip into an array of eight 2×2 optical switches as a variation ofthe fifth embodiment in accordance with the present invention;

FIG. 19 is a schematic plan view showing a configuration incorporatingthe IC bare chip into a PLC chip of a 4×4 matrix optical switch of asixth embodiment in accordance with the present invention;

FIG. 20 is an enlarged view of a duplex type optical switch of FIG. 19constituting an optical matrix switch completely individually drivableby a small number of driving circuits;

FIG. 21 is a circuit diagram showing a concrete circuit configuration ofthe sixth embodiment in accordance with the present invention;

FIG. 22 is a schematic plan view showing a configuration of atransmitter module including a variable optical attenuator of a seventhembodiment in accordance with the present invention; and

FIG. 23 is a schematic plan view showing a configuration of a wavelengthselector of an eighth embodiment in accordance with the presentinvention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The optical module and optical switch constituting the same according tothe embodiments of the present invention will be described below withreference to the drawings.

First Embodiment

A plurality of 2×2 optical switch units constituting a 1×N opticalswitch carry out ON/OFF switching under some constraints rather thanindependently of each other. If the maximum number of stages of a treetype 1×N optical switch is n (1≦n≦log₂N), then a given mth stage fromthe input waveguide includes 2^(m−1)2×2 optical switch units (1≦m≦n).For example, the tree type 1×8 optical switch as shown in FIG. 4includes four 2×2 optical switch units 3-1, 3-2, 3-3 and 3-4 in thethird stage. In the case of an ordinary unicast, only one of the abovefour 2×2 optical switch units is brought into on-state, and only one ofthe N gate optical switches connected to the optical output ports isbrought into on-state.

Thus, let us consider a case where the 2×2 optical switch units aredivided into a plurality of groups, in each of which only one 2×2optical switch unit is brought into on-state, such as each stage of theforegoing example, and where the 2×2 optical switch units in the samegroup share a single driving power supply circuit. In this case, thedriving power supply circuit of each group can supply an optimum voltageto each optical switch unit in the group, without impairing theflexibility by sharing the driving power supply circuit. Although thecomplete sharing can reduce the number of the driving power supplycircuits to a minimum, a partial sharing can offer an effectcorresponding to the number reduced. As for the sharing of power supplylines, it can be implemented on a chip integrating 1×N optical switches,or on a module substrate on which control lines are integrated and thechip integrating 1×N optical switches are assembled.

FIG. 7 shows an arrangement of a first embodiment of a tree type 1×8optical switch in accordance with the present invention. It isimplemented by connecting two2×2 optical switch units to a 2×2 opticalswitch unit connected to an optical input port, and successivelycascading 2×2 optical switch units in a 3-stage constitution. The thirdstage 2×2 optical switch units have their output waveguides connected topredetermined optical output ports via correspondence gate opticalswitches respectively. The 2×2 optical switch units (1-1–3-4) eachconsist of a symmetric type MZI with a symmetric arm length shown inFIGS. 3A–3C. The gate optical switches (G-1–G-8) each consist of anasymmetric type MZI whose arm lengths differ from each other by a halfwavelength. With this arrangement, the 1×8 optical switch can enlargethe extinction ratio when no current is supplied so that the effect ofinterrupting light from the other ports is improved. To control theindividual 2×2 optical switch units, power supply lines 21–24 of thedriving current supply circuits are connected to analog adjustabledriving power supply circuits (not known in FIG. 7), and the drivingcurrent supply circuits are connected to control lines. The power issupplied to one of the two thin-film heaters of the 2×2 optical switchunit. The control lines are not shown in FIG. 7 to simplify the drawing.

FIG. 8 shows some driving current supply circuits of the firstembodiment of the tree type 1×8 optical switch. It is an enlarged viewshowing the 2×2 optical switch units 1-1 and 2-1 of the tree type 1×8optical switch as shown in FIG. 7. A first driving current supplycircuit has its power supply line 21 connected to an analog adjustabledriving power supply circuit 31, and its control line 51 connected to anelectrical digital switch 41. Other driving current supply circuits areconnected likewise. The electrical digital switches 41 and 42 areincorporated into an IC including many transistor circuits integratedtherein, and brought into conduction/interruption by a TTL level input,thereby bringing the 2×2 optical switch units into on-state or out ofoff-state.

It greatly differs from the conventional arrangement in that itcommunizes the driving power supply circuits. For example, the four 2×2optical switch units 3-1–3-4 in the third stage are connected inparallel to the power supply line 23 of the driving current supplycircuit to share the third driving power supply circuit. Likewise, thetwo 2×2 optical switch units 2-1 and 2-2 in the second stage share thesecond driving power supply circuit and the eight gate optical switchesG-1–G-8 share the fourth driving power supply circuit. Thus, the numberof the driving power supply circuits can be reduced from 15 of theconventional module to four. In addition, since the power supply linesof the driving current supply circuits can be shared, the area of wiringcan also be reduced, furthermore, electrical contact terminalsconnecting the optical switch chip with the driving power supplycircuits can be reduced.

Here, a fabrication method of an optical switch chip using silica-basedwaveguides will be described. The optical switch chip associated withthe first embodiment of the 1×8 optical switch is fabricated on asilicon substrate 1 mm thick using the known technology. Thesilica-based waveguides are formed by flame hydrolysis depositiontechnology using a hydrolysis reaction of gas sources such as SiCl₄ andCl₄, and by reactive ion etching technology. The thin-film heaters forlocal heating are formed by vacuum evaporation and etching. The wafer isdiced and fixed to a ceramic substrate, followed by connecting singlemode fibers to the optical input port and optical output ports. Each oneof the thin-film heaters is connected to the driving power supplycircuit and electrical digital switch via an electrical contactterminal, thereby forming the 1×8 optical switch module.

The 1×8 optical switch module also includes an electrical circuitsubstrate mounting a serial/parallel converter for a control signal. Theserial/parallel converter converts control information input as a serialsignal to TTL level parallel signals, and supplies its outputs to 15electrical digital switches.

Thus, the number of the electrical contact terminals of the 1×8 opticalswitch module is about eight including about three serial control signallines for controlling the serial/parallel converter, four power supplylines connected to the four driving power supply circuits and a groundwire. Thus, the present embodiment can also reduce the number of theelectrical contact terminals. The reduction effect increases with thescale of the optical switch chip. For example, as for the 1×128 opticalswitch, the number of the parallel signal lines connected to the 255optical switches are reduced from 255 to about 20. As for the electricalcircuits such as the electrical digital switches, they can be integratednot only into the optical switch module, but also into an outside of theoptical switch module. However, it is preferable that they areintegrated into the optical switch module from the viewpoint of reducingthe number of the electrical contact terminals and of miniaturizing thesystem by the integration.

FIG. 9 shows an embodiment of the tree type 1×128 optical switch modulein accordance with the present invention. The module composed of a 1×128optical switch chip 301 and an electrical circuit substrate 302. The1×128 optical switch chip 301 has on a Si substrate the 2×2 opticalswitch units using symmetric type MZIs, which are formed in a7-stagetree fashion. The 2×2 optical switch units and the gate optical switchesshare the power supply lines 311 of the driving current supply circuitsstage by stage, thereby sharply reducing the number of the power supplylines from 255 to eight, and the area of the substrate.

On the other hand, the control lines 312 of the driving current supplycircuits are connected to Ics 322 a–322 h of the electrical digitalswitches mounted on the electrical circuit substrate 302 via electrodepads 313, gold wires 314 and electrode pads 321. The ICs 322 a–322 h areconnected in series, and can be driven by signals such as a common clocksignal, data signal and latch signal through control signal lines 323.For example, using a 1-MHz clock signal, they each latch a serialsignal, which is assigned the control information about turning on andoff of the individual thin-film heaters on a time base, at every timeframe interval. Such time division multiplexed serial signal isconverted by the serial/parallel converter into parallel signals to besupplied to the 255 electrical digital switches. Thus, all the thin-filmheaters can be driven by the three control signal lines.

The 1×N optical switch thus fabricated can implement a variety offunctions as follows.

(a) Unicast Operation

The output voltage of each driving power supply circuit is adjusted toan optimum operating voltage for the ON-state MZI type optical switch.The electrical digital switch connected to a desired one of the 2×2optical switch units in each stage is brought into conduction (ON) viathe serial control signal lines, with bringing the other electricaldigital switches of the stage out of conduction (OFF). Thus, the lightsupplied to the optical input port can be output from desired one of theoptical output ports. In addition, the extinction ratio can be improvedby bringing the gate optical switch that outputs the light intoconduction with the other gate optical switches being out of conduction.In this case, the insertion loss of the 1×128 optical switch is 3.7 dB,for example, and the extinction ratio thereof is 51 dB, which cansatisfy the desired characteristics.

(b) Broadcast Operation

The 2×2optical switch unit is an MZI type optical switch. Thus, using anintermediate potential between the optimum operation potentials for theconduction and interruption states of one of the outputs enables the twooutputs to produce light at the same intensity. Placing all the 2×2optical switch units at the same state and bringing all the gate opticalswitches into conduction enable all the output ports to bring out theoptical input signal. This operation mode is called a broadcastoperation. Such operation mode utilizes the feature that the opticalswitch is an MZI (Mach-Zehnder interferometer) type, and hence can varythe intensity ratio between the two outputs continuously. Almost allother optical switches such as MEMS (micro electromechanical system)optical switch cannot perform such operation.

(c) Applied Voltage Adjusting Function

As described above, in the unicast operation mode, only one 2×2 opticalswitch unit is brought into conduction (ON) in each stage by thecorresponding 2×2 optical switch unit. Accordingly, even though all the2×2 optical switch units in each stage share the corresponding drivingpower supply circuit, it is connected to only one 2×2 optical switchunit in the operation. Thus, the voltage can be adjusted independentlywhich is to be applied to the 2×2 optical switch unit connected with thecircuit to the electrical digital switch in the conduction (ON) state.

Utilizing the adjusting function of the applied voltage makes itpossible to compensate for the individual variations in the optimumoperating voltage resulting from fabrication errors of the 2×2 opticalswitch units. Usually, the fabricated 2×2 optical switch units have somevariations in their optimum operating voltages because of thefabrication error. Driving the 2×2 optical switch units havingvariations in the optimum operating voltage at the same voltage cancause fluctuations in the insertion loss and extinction ratio, therebydegrading their characteristics. The degradation in the characteristicscan be prevented by measuring the optimum applied voltages to theindividual 2×2 optical switch units in advance and adjusting themindividually.

(d) Equalizing Insertion Loss

The loss variations between the optical output ports of the 1×N opticalswitch can be equalized by adjusting the ON-state losses by utilizingthe feature of the final stage gate optical switches that they canoperate as variable optical attenuators capable of adjusting theiroutput light from transmission to interruption continuously. Asdescribed above, the insertion losses of the optical paths in the 1×Noptical switch can vary slightly because of such as the fabricationerror of the 2×2 optical switch units even though the applied voltagesto the 2×2 optical switch units are adjusted to the optimum operatingvoltages. The value of the variation is about 1 dB for a 1×8 opticalswitch. The loss differences between the optical output ports can bereduced by increasing the losses of the final stage gate switches withsmaller losses such that they are adjusted to the maximum loss on theoptical paths.

As for the two 3-dB couplers constituting the MZI type optical switch,directional couplers are used which are formed by placing two waveguidesside by side in close proximity of a few micrometers. This is becausethe directional couplers have lower insertion loss than other devices.However, the 3-dB couplers are not limited to the arrangement. Otherdevices can be used such as multimode interferometer (MMI) couplersusing multimode waveguides and wavelength independent couplers (WINCs)formed by cascading these couplers.

(Second Embodiment)

FIG. 10 shows an arrangement of a second embodiment of the tree type 1×8optical switch in accordance with the present invention. The secondembodiment differs from the first embodiment in that the 2×2 opticalswitch units and gate optical switches of the second embodiment shareonly one driving power supply circuit. It can implement the foregoing(a) unicast operation and (b) broadcast operation, and can reduce thenumber of the driving power supply circuits from four to one. However,since the voltages applied to the 2×2 optical switch units and gateoptical switches cannot be adjusted individually, the second embodimentdoes not have the foregoing (c) applied voltage adjusting function and(d) equalizing insertion loss, thereby bringing about slightcharacteristic degradation in the insertion loss and extinction ratio.

(Third Embodiment)

FIG. 11 shows an arrangement of a third embodiment of a hybrid type 1×8optical switch in accordance with the present invention. The thirdembodiment is a hybrid type combining a tree type construction and a taptype construction. The 2×2 optical switch units at each stage share adriving power supply circuit, and all the gate optical switches share ananother driving power supply circuit. Thus, the number of the drivingpower supply circuits is six, which is greater than that of the firstembodiment, but is much less than that of the conventional example whichis 15. As the first and second embodiments, the third embodiment canimplement the foregoing (a) unicast operation and (b) broadcastoperation. In addition, it can implement (c) applied voltage adjustingfunction and (d) equalizing insertion loss.

Although the foregoing description is made by way of some concreteexamples, there are a variety of combinations of sharing the drivingpower supply circuits and combining the tree type and tap typearrangements. Table 1 shows some important examples. The number of thedriving power supply circuits is given for the 1×8 optical switch by wayof example.

TABLE 1 optical shared in shared in shared in unshared switch blockblock each stage units shared in shared in unshared gate optical blockblock switches Tree/tap Tree Tap Tree Tap Tree Tap Tree Tap Number 1 1 22 4 9 15 16 of driving power supply circuits function (a) ∘ ∘ ∘ ∘ ∘ ∘ ∘∘ function (b) ∘ x ∘ x ∘ ∘ ∘ ∘ function (c) x x x x ∘ ∘ ∘ ∘ function (d)x x ∘ ∘ ∘ ∘ ∘ ∘ (note) function (a): unicast operation function (b):broadcast operation function (c): applied voltage adjusting functionfunction (d): equalizing insertion loss

All the complete sharing in Table 1 can carry out the unicast operation.Since the conventional example in the rightmost column does not carryout the sharing of the driving power supply circuits, it requires thesame number of the driving circuits as the 2×2 optical switch units.This means that 15 in tree type and 16 in tap type 2×2 optical switchunits are required. In contrast with this, when the 2×2 optical switchunits in the individual stages and the gate optical switches in blockshare the respective driving power supply circuits, the number of thedriving power supply circuits in the tree type (first embodiment) andtap type constructions can be sharply reduced to four and nine,respectively. In addition, they can achieve the foregoing (c) appliedvoltage adjusting function and (d) equalizing insertion loss withoutfail.

Proceeding the sharing further offers a tradeoff between reducing thenumber of the driving power supply circuits and the loss of thefunctions of (c) and (d). However, the optimum arrangement can be chosenconsidering the characteristics and the number of the driving powersupply circuits. Although Table 1 does not refer to the hybrid type ofthe tree type and tap type constructions, the hybrid type can be handledin the same manner because there is no difference between the tree typeand tap type in the function of (c) and (d) on the category of thesharing.

(Fourth Embodiment)

FIG. 12 shows an arrangement of a fourth embodiment of the tree type 1×8optical switch in accordance with the present invention. The firstembodiment is configured such that it supplies power to only one of thetwo thin-film heaters of each 2×2 optical switch unit. In contrast withthis, the fourth embodiment supplies power to the two thin-film heaters.The control of the individual 2×2 optical switch units is carried out byconnecting the analog adjustable driving power supply circuits to firstends of the power supply lines 71–74, and control lines (not shown inFIG. 12) to second ends thereof.

FIG. 13 shows a configuration of the driving current supply circuits ofthe fourth embodiment of the tree type 1×8 optical switch. FIG. 13 is anenlarged view showing the 2×2 optical switch units 1-1 and 2-1 of thetree type 1×8 optical switch as shown in FIG. 12. The power supply line71 of the driving current supply circuit has its first end connected tothe two thin-film heaters and its second end connected to the analogadjustable driving power supply circuit 31. As in the first embodiment,the 2×2 optical switch units in each stage share one of the drivingpower supply circuits. A control line 81 a of the driving current supplycircuit is connected to the electrical digital switch 41 a, and acontrol line 81 b of the driving current supply circuit is connected tothe electrical digital switch 41 b to drive the thin-film heatersindividually. The same configuration is applied to the other drivingcurrent supply circuits.

The configuration can expand the adjustment of the individually appliedvoltages in both the positive and negative directions in the foregoing(c) applied voltage adjusting function. The symmetric type MZI usuallyhas the least crosstalk to the bar path when no voltage is applied tothe thin-film heaters. However, the optimum point can be shifted by thefabrication error of the 2×2 optical switch unit and the like. Thepresent embodiment can apply the optimum voltage at the minimum powerconsumption, even if the optimum point is shifted in either the positiveor negative direction in the state without applying the voltage.

Providing heaters at both sides of the arm waveguides individuallyenables one of the thin-film heaters to serve as a spare heater. Forexample, even if one of the thin-film heaters of any one of the 2×2optical switch units damages, the present embodiment can continue theoptical switch operation successfully because the other thin-film heatercan be driven independently. Thus, the present embodiment can sharplyreduce the probability of the failure of the 1×N optical switch.

On the other hand, the number of the electrical digital switches isdoubled, which offers a trade off of doubling the number of the drivingcurrent supply lines although they are short length, thereby increasingthe area of the wiring section in the upper portion of the substrate.However, the present embodiment can minimize the increase in the area ofthe wiring section by sharing the driving power supply circuits andpower supply lines together with adopting the ICs including theelectrical digital switches.

Although the fourth embodiment is described by way of example thatdrives the thin-film heaters on both arms of the 2×2 optical switchunits in the first embodiment, the same structure is applicable to otherarrangements including the second to fourth embodiments. In addition,although the foregoing embodiments have the multi-stage arrangements ofthe 2×2 optical switch units, other multi-stage arrangements arepossible using other two-branching optical switches such as 1×2 opticalswitches.

(Fifth Embodiment)

First, the common basic principle of the fifth to eighth embodiments ofthe optical module in accordance with the present invention will bedescribed.

The inventors of the present invention have found the following factsabout the electrical circuits for driving the heaters as shown in FIG.5. First, the optical switch units that are brought into conduction(on-state) in operation are subject to certain constraints rather thanto a random scheme. Second, the driving power supply circuits can becombined and shared without or nearly without losing the flexibility byorganizing the constraints.

The facts will be described by way of example of the driving powersupplies shared in the tree type 1×N optical switch. As describedbefore, assume that the 1×N optical switch is composed of the 1×2optical switch units which are used as a basic optical switch unitindividually and arranged in an n-stage tree fashion. In this case, onlyone optical switch unit is brought into conduction (current drivingstate) in a given stage a (1≦a≦n), such as one of the four opticalswitch units 3-1, 3-2, 3-3 and 3-4 in the third stage. In addition, onlyone of the N gate optical switches on the output waveguides is broughtinto conduction (current driving state).

When the 1×2 optical switch consists of the MZI optical switch circuitas shown in FIGS. 3A–3C, an example of the electrical circuit fordriving it is given by FIG. 6. As shown in FIG. 6, the heaters havetheir first ends connected to the driving power supplies (drivingcircuits) 31 and 32 capable of analog outputs, and their second endsconnected to the electrical digital switches 41 and 42. The outputs ofthe analog power supply circuits 31 and 32 are individually adjusted atthe optimum driving voltages for the optical switch units 1-1 and 2-1previously. On the other hand, the electrical digital switches 41 and42, which consist of an IC (integrated circuit) including multipletransistor circuits, are brought into conduction or out of conduction inresponse to TTL level inputs. As in the 1×8 optical switch shown in FIG.7, the optical switch units of each stage share the analog drivingcircuit 31 or 32.

FIG. 8 shows the circuit arrangement of FIG. 7 in more detail. As shownin FIG. 8, each stage can share the analog driving circuit 31 or 32without losing the individually adjusting function that enables theanalog driving circuits 31 and 32 to apply individually adjustedvoltages to the optical switch units 1-1 and 2-1. This is because onlyone optical switch is brought into conduction in each stage all at once.Thus carrying out the sharing on the PLC substrate can reduce the areaof the electrical wiring region on the substrate, and reduce the numberof the connection wires from the PLC substrate to the outside and thenumber of the analog driving circuits.

However, as the scale of the optical switch increases recently, furtherminiaturization is necessary by reducing the electrical wiring on thesubstrate. Let us return here to the example of 1×128 optical switch ofFIG. 5. The driving signal power supply lines 507 in the upper side ofFIG. 5 are connected to the analog driving circuits 31 and 32 on theanalog feeding side, and the heater driving wires 523 in the lower sideare connected to the electrical digital switches 41 and 42. As for theanalog feeding side of the unit optical switches 503, each stage canshare them in accordance with the scheme as described above withreference to FIGS. 7 and 8.

In contrast, as for the electrical digital switch side of the opticalswitch units 503, they are pulled to the substrate edge by the goldelectrical circuits 509, connected to the external substrate 521 viawires 513 that connect the gold wiring electrode pads 511 and 515, andthen connected to the driving ICs 525. The electrical circuit includingthe ICs 525 is configured as shown in FIG. 14 in the present invention.The tree type 1×128 optical switch 501 of FIG. 5 includes 255 opticalswitch units 503 (denoted by #1–#255 in FIG. 14). The optical switchunits 503 have their individual heaters 907 connected to four 64-bitdriving ICs 525-1–525-4 as illustrated in FIG. 14. The driving ICs525-1–525-4 are a CMOS IC (CMOS integrated circuit) including a shiftregister and a latch (not shown), and have both functions of the digitalswitches 41 and 42 and the serial/parallel converter (not shown). Thedriving IC 525-1–525-4 can be extended to 64 bits ×4=256 bits bysynchronizing each other by connecting in cascade as shown in FIG. 14.

Consider a case where a 1-MHz clock signal is used for driving the ICs525-1–525-4 as illustrated in FIG. 15. The ON/OFF control of each of theheaters 907 is assigned to individual time base in response to eachpulse of the clock signal. Thus, the individual digital switch (41 ofFIG. 6) can be turned on and off with some switching speed within theperiod of 0.26 ms of the latch pulse. Since the switching speed of thethermooptic switch, 2 ms typical, is much slower than the clock signal,the ON/OFF control of the 256 heaters 907 can be carried out in responseto the three input signals with respect to the input and output of theICs 525-1–525-4.

Although the example of FIG. 15 uses the clock signal with a rate of 1MHz, using a clock signal with a rate of 10 MHz will enable 2,560heaters to be switched by latching at every 0.26 ms. Thus selecting theclock rate appropriately makes it possible to control a lot of heaters.

In addition, according to the present invention, the driving ICs525-1–525-4 are mounted on the PLC substrate as small type bare chipswith a size of 2 mm×7 mm, for example, as will be described later withreference to FIG. 16. Thus, the wiring between the heaters and the ICson the PLC substrate can be integrated to reduce from 255 to only threesignal lines. Therefore, the present embodiment can sharply reduce thearea of the electrical wiring, and hence the area of the PLC substrate.

Furthermore, according to the present invention, since the driving ICsare mounted on the PLC substrate, the number of wires to be pulled outof the PLC substrate is sharply reduced to 20. As a result, the numberof wires between the PLC substrate and the electrical circuit substrate,on which the electrical driving circuits are mounted, can be reduce to20, thereby being able to greatly reduce the number of the wire bondingsfrom 510 to 20.

Furthermore, according to the present invention, the module can beminiaturized greatly because the driving IC assembly substrate isremoved.

Moreover, according to the present invention, the number of theelectrodes of the probe for the inspection at the PLC substrate levelbefore assembling the module in the fabrication process can be reducedfrom 510 to 20 because the driving ICs are mounted on the PLC substrate.Therefore the present embodiment can simplify the electrical probe andthe device for aligning the probe.

FIG. 16 is a schematic plan view showing a fifth embodiment of theoptical module in accordance with the present invention. The presentembodiment is an example of a 1×128 PLC thermooptic switch in accordancewith the present invention, in which the driving ICs are mounted on thePLC substrate. In FIG. 16, the wiring of the optical switch units in thefourth and later stage is omitted for the simplicity of the drawing. InFIG. 16, the reference numeral 101 designates the 1×128 PLC thermoopticswitch (1×128 optical switch). Each reference numeral 103 designates a1×2 optical switch, 104 designates an optical waveguide, 105 designatesa gate optical switch, 107 designates a heater driving gold electricalcircuit, 109 designates a basic switch driving IC (bare chip)(corresponding to the driving ICs 525-1–525-4 of FIG. 14), 111designates a gold electrical circuit for control signal, 113 designatesan electrode pad, 115 designates a gold wire, 117 designates anelectrode pad, and 119 designates a driving signal power supply line(power supply line used by the driving power supply circuit).

The 1×2 optical switches 103 and optical waveguides 104 of the presentembodiment have the same arrangements as their counterparts of FIGS.3A–3C. In the 1×128 PLC thermooptic switch 101, the optical waveguides104 are formed on the Si substrate by the flame hydrolysis depositionmethod. The heaters 907 are formed by patterning a Ta₂N film. Inaddition, the electrical wiring 107 of the heaters are formed bypatterning a gold thin film on the Si substrate.

The present embodiment differs from the conventional example in thefollowing electrical wiring structure. Specifically, as shown in FIG.16, the present embodiment shares the power supply lines 119 of thedriving power supply circuits on the PLC substrate to reduce the area ofthe electrical wiring, and mounts the heater driving ICs 109 on the PLCsubstrate in the form of bare chips.

Each heater driving IC bare chip 109 is fixed to the PLC substrate withelectrode pads 203 and 204 facing upward as illustrated in an enlargedview of FIG. 17. The first electrode pads 203 are connected to theheaters of the optical switch units 103. The second electrode pads 204are connected to the gate control signals. The second electrode pads 204include from left to right in FIG. 17, a GND terminal, a clock terminal,a latch terminal and a signal terminal, and gold thin film wires 111(shown in FIG. 16) are pulled out from the second electrode pads 204.

Then, the electrode pads 201 and 202 on the PLC substrate are connectedto the electrode pads 203 and 204 on the heater driving IC 109 by thegold wires 205 and 206. In addition, the IC 109 and gold wires 205 and206 are subjected to resin molding by silicone resin (not shown) toimprove their reliability. Such an assembly of the IC bare chip isgenerally formed on the electrical wiring substrate, and can ensure thereliability unlike the wire bonding between the substrates as shown inFIG. 5.

Take notice here that an optical waveguide circuit substrate of alarge-scale optical switch must include many, say, up to several hundredphase shifters (heaters) dispersed in a broad region thereon. However,it is very difficult to carry out the fine electrical wiring process onthe optical waveguide circuit substrate. Thus, fabricating the drivingelectrical circuits of the phase shifters (heaters) on the samesubstrate (optical waveguide circuit substrate) presents a new problemdifferent from fabricating the conventional electrical circuit andintegrated circuit.

The present embodiment solves the new problem by mounting the gates ofthe driving ICs on the Si substrate and by grouping (sharing) them asshown in FIGS. 14 and 15. Thus, it should be noted that the presentinvention not only mounts the driving ICs on the Si substrate. Forreference purposes, there has been no report to date that driving ICs orcontrol ICs of an optical switch are assembled on the PLC substrate.

As for the driving circuit using the ICs, it can drive 256 heaters 907simultaneously through three heater control signals consisting of theclock, latch and signal (data) as described above with reference toFIGS. 8, 14 and 15.

With the foregoing arrangement, the present embodiment can offer thefollowing advantages.

(1) It can reduce the area of the electrical wiring on the PLCsubstrate.

(2) It can obviate the need for the IC assembly substrate, thereby beingable to reduce the module size.

(3) It can reduce the number of wire bondings between the PLC substrateand the electronic circuit substrate.

(4) It can simplify the inspection process.

These advantages will be described below.

(1) Reduction in the Area of the Electrical Wiring on the PLC Substrate.

In the conventional example as shown in FIG. 5, since all the heaterdriving gold electrical circuits must be pulled to the electrode pads atthe substrate edge, the area of the electrical wiring region increasesby just that much, thereby increasing the size of the entire substrate.In contrast with this, in the present embodiment of FIG. 16, since thedriving IC gates are mounted and grouped on the PLC substrate, the 255heater wires are only pulled to their nearest driving ICs. Thus, thelength of the electrical wiring is shortened, and the area of theelectrical wiring on the PLC substrate is greatly reduced.

The reduction in the area of the wiring will be estimatedquantitatively. The Ta₂N film heaters must pass a current of 60 mA atthe maximum, for example. Accordingly, the gold electrical circuitspreferably consist of about 50 μm wide electric wiring in a singlelayer, considering that it will be necessary to pattern them on a warpedoptical waveguide substrate that will hamper the fabrication process.The spacing between the wiring must be secured about 50 μm, as well.Estimating the area required for developing the electrical wiring underthe foregoing conditions results in the total wiring width of 25.6 mmfor the 256 wiring. As a typical example, assume that the wiring lengthfrom the individual heaters to the heater driving ICs is 15 mm at theaverage, and the wiring length when the heater driving gold electricalcircuits are pulled to the substrate edge without the heater driving ICis 60 mm at the average. Then the area of the electrical wiring regionfrom the heaters to the digital switch side is reduced to a quarter.

The reduction in the area of the electrical wiring is independent of theadvantages of the foregoing (2) and (3). Accordingly, it is enough forthe heater driving ICs 109 to be placed on the optical waveguidesubstrate in such a manner that the area of the electrical wiring regionbecomes minimum, or the wiring becomes shortest.

FIGS. 16 and 5 are schematic views that cannot show a detailedelectrical wiring layout. As for the conventional example of the 1×128optical switch as shown in FIG. 5, the area of its optical waveguidesubstrate is 57 mm×60 mm, and the miniaturization of the PLC substrateis limited by the electrical wiring region. In contrast with this, thepresent embodiment can reduce the area of the electrical wiring regionto a quater, thereby being able to miniaturize the PLC substrate to 30mm×60 mm.

(2) Reduction in Module Size Because of the IC Assembly Substrate isMade Redundant.

It is obvious that assembling the heater driving ICs on the PLCsubstrate can reduce the module size by an amount corresponding to theIC assembly substrate section.

(3) Reduction in the Number of Wire Bondings Between the PLC Substrateand the Electronic Circuit Substrate.

Since the present embodiment assembles the heater driving ICs on the PLCsubstrate, it can sharply reduce the number of wire bondings between thetwo substrates.

In contrast to about 510 wires between the substrates in theconventional example, the present embodiment can reduce it to 20,including three signal lines and 17 power supply lines and the like. Thenumber of wires of 20 or so can completely avoid the expansion of thewidth of the wire connections between the substrates such as a few tensof millimeters in the conventional example, thereby being able toprevent the degradation in the reliability. The present embodiment cannarrow the width of the wire bonding region to 2.8 mm, and can establishconnections individually with cables without using the wires. Thus, itcan completely solve the problems involved in the wire bonding.

(4) Simplification of the Inspection Process.

In the present embodiment, an inspection is carried out to check as towhether a product with desired characteristics has been made at the stepof the PLC substrate in the actual module fabrication process bybringing the electrical probe into contact to drive the heaters from theoutside. In the conventional example, it is necessary to bring theelectrical probe into contact with the 510 electrode padssimultaneously, which demands an expensive special electrical probe andaligning equipment enabling the electrical probe to make contact at highparallelism. In contrast with this, the present embodiment as shown inFIG. 16 can reduce the number of the electrode pads to be brought intocontact to 20, which enables an inexpensive electrical probe to makecontact with the electrode pads easily, thereby facilitating theinspection. Thus, it can sharply reduce the members, cost of equipmentand time required for the inspection process.

In summary, the present embodiment assembles the bare chips of theoptical switch driving ICs directly on the PLC substrate. Accordingly,it can sharply reduce the electrical wiring region, miniaturize the PLCsubstrate, greatly reduce the number of the gold wires between thesubstrates from 256 to 20, and increase the yield and reliability. Inaddition, the present embodiment can sharply reduce the module size bythe amount corresponding to the removal of the driving IC assemblysubstrate. Furthermore, it can simplify the probe and equipment requiredfor the inspection process, thereby being able to reduce its cost andtime.

The present invention is applicable not only to a single large scaleswitch as shown in FIG. 16, but also to an integrated circuit includingeight array of 2×2 optical switch units on a same PLC substrate, whichis used as an optical add/drop multiplexer/demultiplexer, for example,offering the same advantages as described above. FIG. 18 shows anexample of such an arrangement. In FIG. 18, the electrical wiringbetween the optical switch units 103 and IC 109 are omitted for the sakeof simplifying the drawing.

The present invention is also applicable to a variable opticalattenuator and its arrayed module, a dispersion compensator, and a gainequalizer with offering similar advantages.

Furthermore, it is obvious that the present invention is applicable tooptical switch and the like using an optical circuit other than the PLCsubstrate with offering the same advantages as the case of using the PLCsubstrate. For example, the present invention is applicable to anoptical switch using LiNbO₃ (LN) optical waveguides with offering thesame advantages as the case of using the PLC substrate.

(Sixth Embodiment)

FIG. 19 shows an arrangement of a 4×4 optical matrix switch of a sixthembodiment in accordance with the present invention. The 4×4 opticalmatrix switch 401 includes four input waveguides 407 and four outputwaveguides 409, which intersect in a grating fashion at 16 locations,and duplex type optical cross-point switches 411 placed at theindividual intersections.

FIG. 20 is an enlarged view showing the duplex type optical cross-pointswitch 411. The duplex type optical cross-point switch 411 includes a1×2 optical switch 415, a 2×1 optical switch 416 and an intra-unitwaveguide 421 connected between them. In addition, as shown in FIG. 19,the vertical four driving signal power supply lines 403 connect the 1×2optical switches 415 on the individual lines to corresponding driving(power supply) circuits shared by the 1×2 optical switches. Furthermore,the horizontal four driving signal power supply lines 405 connect the2×1 optical switches 416 on the individual lines to correspondingdriving (power supply) circuits shared by the 2×1 optical switches.

FIG. 21 shows a concrete arrangement of the duplex type opticalcross-point switch 411 and electrical control circuit. In FIG. 21, thereference numeral 401 designates the optical matrix chip, 411 designatesthe duplex type optical switch unit, 601 designates a driving (analogpower supply) circuit shared by the 1×2 optical switches, 602 designatesa driving (analog power supply) circuit shared by the 2×1 opticalswitches, and each reference numeral 603 designates an electricaldigital switch.

The 1×2 optical switch 415 and 2×1 optical switch 416 constituting theduplex type optical cross-point switch 411 each employ an MZI opticalswitch having an optical path difference of half a wavelength as theconventional example. In the individual MZI optical switches,thermooptic heaters 617 and 618 on the arm waveguides with a shorteroptical path have electrical wiring connected thereto. The first ends ofthe electrical wiring are connected to the electrical digital switches603, and the second ends of the electrical wiring are connected to thedriving power supplies (driving circuits) 601 and 602 capable of analogoutput. The electrical digital switches 603 are each incorporated intoan IC including many transistor circuits integrated therein so that theTTL level input can bring them into conduction or out of conduction.

The present embodiment differs noticeably from the conventionalarrangement in the following points as shown in FIGS. 19 and 20.

(1) The analog driving circuits are shared by the 1×2 optical switchesfor each output waveguide, and by the 2×1 optical switches for eachinput waveguide.

(2) The heater driving IC is mounted on the PLC substrate as a barechip.

The different points will be described in more detail.

Referring to FIGS. 19 and 20, (1) sharing of the analog driving circuitswill be described. The shared driving circuit 1 a is connected to fouroptical cross-point switches SW_(x,1) in parallel, each of whichconsists of a 1×2 optical switch, where ×=1–4. Likewise, the shareddriving circuits 2 a–4 a are connected to the optical cross-pointswitches SW_(x,2)–SW_(x,4) in parallel, respectively. In addition, theshared driving circuit 1 b is connected to four optical cross-pointswitches SW_(1,y) in parallel, each of which consists of a 2×1 opticalswitch, where y=1–4. Likewise, the shared driving circuits 2 b–3 b areconnected to the optical cross-point switches SW_(2,y)–SW_(4,y) inparallel, respectively.

Thus, the number of the driving circuits can be reduced from 16 requiredfor the individual connections to eight. In addition, since the wiringto the driving circuits is partially shared by each driving circuit usedin common, it is possible to reduce the area of the wiring, and toreduce the electrical contact terminals from the switch chip alltogether.

Furthermore, (2) assembling the bare chips of the heater driving IC 109on the PLC substrate differ from the conventional example. The assemblyis implemented as described in connection with FIG. 17. In FIG. 19, theIC bare chip 109 is connected to the 32 MZI circuits individuallythrough the heater driving wiring, and operates as the electricaldigital switches 603 to bring the MZI circuits into conduction or out ofconduction. As for an M×L matrix switch, where M and L are integersequal to or greater than three, it can offer advantages similar to theadvantages (1)–(4) of the fifth embodiment as described in detail in thefifth embodiment. For example, although there are 32 electrical digitalswitches 603 corresponding to the 32 optical cross-points, the number ofconnections between the heaters 618 and the IC 109 is reduced to three.Accordingly, the area of the electrical wiring region is decreased andthe number of wires between the substrates is reduced concurrently.Although the description is made by way of example of 4×4 matrix switchhere, such above reduction effect increases with the scale of theoptical matrix switch chip.

As for the two 3-dB couplers (see 1102 a and 1102 b of FIGS. 3A–3C) inthe present MZI optical switch, directional couplers are employed whichare each constructed by placing two waveguides side by side in closeproximity of about several micrometers. This is because the directionalcouplers have an insertion loss smaller than other means. However, the3-dB couplers are not limited to the arrangement, allowing other means.For example, it is certainly possible to use multimode interferometer(MZI) couplers using multimode waveguides, or wavelength independentcouplers (WINC) constructed by cascading these couplers.

(Seventh Embodiment)

FIG. 22 shows a transmitter module including a variable opticalattenuator using an MZI circuit formed on a silica-based PLC substrateas a seventh embodiment in accordance with the present invention. Thetransmitter module comprises the following components. In the left-sideof this figure, a plurality of laser diodes 701 for WDM signals isdisposed for wavelengths λ₁−λ_(N). The laser diodes 701 have theiroutputs connected through optical fibers 703 to Mach-Zehnder typevariable optical attenuators 707 using an MZI circuit fabricated from asilica-based PLC and the arrayed module of the attenuators 707 isfabricated on a PLC substrate 705. The variable optical attenuators 707have their outputs connected to an arrayed waveguide grating (AWG) 717through optical fibers 715. The AWG 717 is formed on a PLC substrate(optical waveguides of the AWG are omitted from the drawing).

The MZI circuit incorporates a bare chip IC 709 for driving the variableoptical attenuators 707 on the same substrate. Here, the referencenumeral 711 designates a gold electrical circuit for driving heaters,and 713 designates a gold electrical circuit for driving the IC 709. Thevariable optical attenuators 707 consist of the MZI circuits(Mach-Zehnder interferometer type optical switches) described before inconnection with FIGS. 3A–3C, and are operated by a continuous current inan analog fashion. Incidentally, the driving IC 709 controls not onlythe gate switches, but also the amount of the current to be passedthrough the variable optical attenuators 707. The amount of the currentto be passed in an analog manner is controlled by a control signalproduced by digitizing the corresponding current amount. The IC used forthe control can be a single chip or a combination of a plurality ofchips as long as it can carry out the analog control of the individualvariable optical attenuators 707. As for the PLC substrate 705 includingthe variable optical attenuators 707, the electrical wiring on thesubstrate is integrated into the IC mounted as in the foregoing fifthembodiment, thereby being able to offer the same advantages as theadvantages (1)–(4) enumerated in the fifth embodiment.

Although the present embodiment is described by way of example of thevariable optical attenuator using a silica-based PLC, the presentinvention is applicable to optical waveguide circuits using othermaterials. In addition, the type of the optical circuit is not limitedto the MZI. For example, it is obvious that a circuit using a Y branchcan also be employed.

(Eighth Embodiment)

FIG. 23 shows a wavelength selector for selecting a given wavelengthfrom a multi-wavelength optical signal as an eighth embodiment inaccordance with the present invention. The wave length selector is aPLC-LN type optical switch fabricated by joining a PLC substrate and anLN optical waveguide substrate at their ends. In FIG. 23, the referencenumeral 801 designates a first substrate composed of a PLC including anarrayed waveguide grating, 803 designates a second substrate composed ofLiNbO₃ (LN), and 805 designates a third substrate composed of a PLCincluding an arrayed waveguide grating. The three substrates are joinedin this order at their ends by a bonding adhesive. The reference numeral809 designates a driving IC mounted on the second substrate 803. Eachreference numeral 811 designates a LiNbO₃ optical waveguide formed onthe LN substrate 803. The reference numeral 813 designates a heaterdriving gold electrical circuit, and 815 designates an IC control goldelectrical circuit.

In the present embodiment, arrayed waveguide gratings (AWGs) are formedon the PLC substrates 801 and 805, though the optical waveguides such asthe AWGs are not shown in FIG. 23. The MZI circuits 807 are eachcomposed of a coupler on the PLC substrate 801 and the LN opticalwaveguides 811 on the LN substrate 803. The phase variation of half awavelength required for the optical switching is given by theelectro-optic effect by the voltage applied to the heater on the LNoptical waveguide 811. The example uses the MZI optical switch as anON/OFF type optical gate switch.

An N-wavelength WDM signal (wavelengths λ₁, λ₂, λ₃, . . . and λ_(N))input via the optical input port at the left-hand side of FIG. 23 isdemultiplexed into individual wavelengths by the AWG on the first PLCsubstrate 801. Subsequently, the individual wavelengths are passedthrough the MZI circuits 807 on the second substrate 803 at the centerso that only one or more desired wavelength are selected by the gateswitch function of the MZI circuits 807. Then, the selected one or morewavelengths are multiplexed again by an output side AWG (not shown inFIG. 23) on the third PLC substrate 805. As just described, the PLC-LNtype optical switch of the present embodiment functions as a wavelengthselector for selecting any desired wavelengths from the wavelengthdivision multiplexed optical signal. FIG. 23 illustrates an example thatselects only the optical signal with the wavelength λ₂.

A combination of the silica-based PLC substrate and an optical waveguidecircuit composed of other materials as the present embodiment offers thefollowing advantages. It can utilize an extensive menu of opticalcircuits such as AWGs implemented by PLC substrates. In addition, thePLC substrate has a smaller radius of curvature of an optical waveguidewith proven performance than the other material's substrate, therebyenabling fabrication of a high performance circuit with a smaller size.Furthermore, as an optical switch, using the LN substrate offers anadvantage of being able to implement a high speed optical switching atlow power consumption.

The circuit including an optical switch having a combination of thePLC-LN substrates can offer the advantages similar to the advantages(1)–(4) described in the fifth embodiment by directly mounting the TC onthe optical circuit substrate.

Although the present embodiment is described by way of example of thewavelength selector, other optical circuits can offer similaradvantages. For example, utilizing the structure of the presentembodiment makes it possible to arrange a tree-type optical switch suchas a 1×8 optical switch similar to that of FIG. 16. In this case, the1×8 optical switch offers the advantages similar to the advantages(1)–(4) described in the fifth embodiment.

The present invention has been described in detail with respect topreferred embodiments, and it will now be apparent from the foregoing tothose skilled in the art that changes and modifications may be madewithout departing from the invention in its broader aspect, and it isthe intention, therefore, in the appended claims to cover all suchchanges and modifications as fall within the true spirit of theinvention.

1. A 1×N optical switch comprising: a two-port optical switch connectedto an input waveguide, and capable of continuously adjusting its opticaloutput intensity from a first output to a second output; a switchsection including one or more two-port optical switches connected incascade to the output(s) of said two-port optical switch, and having itsoutputs connected to N output waveguides, where N is an integer equal toor greater than three; a plurality of gate optical switches, each ofwhich is connected to one of the output waveguides and capable ofcontinuously adjusting its optical output from transmission tointerruption; a plurality of switch driving power supply circuits fordriving said two-port optical switches, said two-port optical switchesbeing divided into groups, each of which includes only one two-portoptical switch that is brought into conduction at a time, and thetwo-port optical switches in a same group sharing one of said switchdriving power supply circuits; a gate driving power supply circuit fordriving said gate optical switches, all the gate optical switchessharing said gate driving power supply circuit; and electrical digitalswitches, each of which is connected to one of said two-port opticalswitches or said gate optical switches for interrupting a drivingcurrent from one of said driving power supply circuits.
 2. The 1×Noptical switch as claimed in claim 1, wherein power supply lines areshared which connect said two-port optical switches to said switchdriving power supply circuits shared, and wherein power supply lines areshared which connect said gate optical switches to said gate drivingpower supply circuit shared.
 3. The 1×N optical switch as claimed inclaim 2, wherein said power supply lines are shared on an optical switchchip in which said switch section, said plurality of gate opticalswitches and said power supply lines are integrated.
 4. The 1×N opticalswitch as claimed in claim 1, wherein each of said two-port opticalswitches has its first output connected to one of said output waveguidesand its second output unconnected or connected to an input of one ofother two-port optical switches to make said switch section a taparrangement.
 5. The 1×N optical switch as claimed in claim 1, whereinsaid two-port optical switches or said gate optical switches are eachcomposed of a 2×2 optical switch having one of its input/output portsunconnected.
 6. The 1×N optical switch as claimed in claim 1, whereinsaid two-port optical switches or said gate optical switches eachconsists of an optical switch using silica-based optical waveguides.